How can I resize my chroma key mask?
The size of the output image  is determined by the image connected to the background.



If you plug in a scaled (or differently sized) image as foreground, it will mess up the sizing of the composite.

In this case your setup is correct but the keying node is filling the frame with a value of black information (0) around the image, and a value of 1 in the alpha channel, creating full opacity.



The way to fix this is making the Black clip be a value that is not 0, so that the alpha channel becomes transparent to that value.
Changing the value to 0.00001 will make the transparency come back: